In many water supply sources a great increase in the nitrate content above 50 mg/l NO.sub.3.sup.- /1 has been recorded, caused inter alia by over-employment of fertiliser to agricultural land and inappropriate disposal of animal sewage. After the connection between high nitrate values, and e.g., cancer of the digestive organs became known, new standards were worked out for 1985 by the European Community, which for nitrates permit an index of 25 mg/l NO.sub.3.sup.- /1 and a highest permissible concentration of less than 50 mg/l NO.sub.3.sup.- /1 in drinking water. Hence in many cases treatment cannot be avoided to ensure lower nitrate concentrations in drinking water.
In the article "Removal of nitrate from drinking water by means of biological denitrification" by Ch. Frank and W. Dott, Bonn, published in Biotech.--Forum--internationale Zeitschift fur Biotechnologie, 3/4, 84, pages 53-57, investigations of such biological processes are described. In these processes bacteria were fixed on the most varied carriers, such as surface growth (adsorption) with nutrient material, percolating filters, immersion bodies, pumice, active carbon, expanded polystyrene balls, foam, (aerated plastics) etc., and used in appropriate reactors in water and waste water treatments.
Two different biological processes should be distinguished:
Lithotrophic (also known as autotrophic) denitrification, in which the addition of hydrogen or elemental sulphur as an electron donor is necessary. Here the dissolved carbon dioxide present in water serves as the source of carbon.
Organotrophic (also known as heterotrophic) denitrification, in which the addition of a carbon carrier such as, e.g., methyl or ethyl alcohol, sugar, molasses, acetic acid, methane etc. is necessary.
In the biological process hitherto used in practice dosing with methanol or ethanol is preferred owing to the shorter reaction time. With this approach, under certain operating conditions (e.g. overdosing or too short a reaction time) undesired by-products such as, for example, formaldehyde and acetaldehyde can be formed, which are classified according to the MAK (maximum work-place concentration) -value guidelines as substances which may be carcinogenic. Hence these processes require expensive control and monitoring systems, which present additional treatment requirements and expenditures.
If instead other carbon carriers such as, for example, sugar, molasses or acetic acid are added, then although according to the present state of knowledge no suspicious reaction products are formed, nevertheless because of long reaction times such processes require larger reaction volumes, which mean a substantial expenditure on plant and investment.
Furthermore all known systems for the denitrifying bacteria used for biological denitrification consist of carriers with surface growth (adsorption), and are therefore susceptible to toxic matter such as, for example, metal ions (chromium, iron, copper, etc.) and require an additional pretreatment step.
Finally the dosing of the water with a source of carbon required in the process necessarily leads to an increase in the bacteria present in the water. Hence such processes are to be judged not only by the reduction of the nitrate loading. The changed bacteriological situation in the purified water delivered and the absolutely necessary after-treatment by, for example a multilayer filter plant, must also be taken into consideration. Here, besides the quantitative effect of the bacterial count, the question of the kinds present comes to the fore. Thus it has been found that after the denitrification reactor, bacterial counts of the order of 10.sup.4 to 10.sup.5 /ml are present in the water, which can only be reduced by after-treatment, e.g. filtration with intermediate aeration, to the order of 10.sup.2 to 10.sup.3 /ml usual in treated water.